The present invention relates to a subscriber line access circuit for use in a telecommunications system. The subscriber line access circuit has a power supply bridge in the form of a first resistor inserted in the b-wire (ground potential side) of a subscriber line, a second resistor inserted in the a-wire (supply potential side) of the subscriber line, and a capacitor which bridges the two wires at the terminals of the two resistors on the sides thereof opposite to the respective potential sources. This supply bridge is coupled on the capacitor side with a two-to-four wire hybrid transformer for the two-wire/four-wire transition to the system, and includes an evaluating circuit which delivers an indicating signal when the subscriber actuates a ground key.
Subscriber line access circuits of this kind--that is, those which contain a power supply bridge individual to the subscriber--are provided in exchanges which contain an electronic coupling field. Here, just as in exchanges with space multiplex coupling fields, it is necessary to guard against an overload of the switching elements located in the remaining portion of the power supply circuit when the ground key actuation exceeds a certain length of time. However, with electronic coupling fields this cannot be done by switching off the associated set subassembly in the case of such a ground key actuation. Overdimensioning the power supply bridge, a conceivable solution to the problem of making the bridge less susceptible to overloads, would lead to the result that only few subscriber line access circuits could be combined in one subassembly.
In addition to the desirability of avoiding overloads in the supply circuit, it is of interest also to be able to recognize insulation faults in the access circuit in order to take appropriate measures for disconnection.
It is therefore the object of the present invention to design a subscriber line access circuit in such a way that the above mentioned problems can be solved at minimum expense.